The advanced four-man Containment and Emergency Response Team
(CERT) that usually guards the nearby US Capitol complex was one
of the first armed teams on the scene to aid municipal police
officers. The CERT unit was quickly told to leave the scene by
Capitol Police command.

Former Navy reservist and tech contractor Aaron Alexis, the
suspect in the shooting that left 12 people dead on Monday
morning , had the proper security clearance to enter Building 197
– the headquarters for Naval Sea Systems Command - at around
12:15 GMT.

Alexis, armed with a shotgun and pistol he took from a security
officer he shot, began firing into a building atrium shortly
after his arrival.

Sources told BBC News the CERT received notice of the shooting
around 12:20 GMT and arrived at the scene a few minutes later.
They were wearing full tactical gear and were armed with HK-416
assault weapons.

A Capitol Police source said the CERT was told by an officer of
the Metropolitan Police Department, the District’s local police
force, that it was the most advanced and equipped response team
at the scene upon arrival.

Nevertheless, the unit’s commander ordered them to leave the
scene. The suspect was eventually killed at 13:00 GMT.

"I don't think it's a far stretch to say that some lives may
have been saved if we were allowed to intervene," a Capitol
Police source said.

A spokeswoman for the Metropolitan Police Department said claims
the CERT was on the scene and later departed were "not
true.”

In addition, the Capitol Police Board is forming a “Fact
Review Team” run by a former US Secret Service official and
aimed at investigating the claims.

Officials are also probing radio logs from Monday morning and
interviewing officers involved, according to Senate
Sergeant-at-Arms Terry Gainer, who has oversight of the Capitol
Police.

"It's a very serious allegation and inference to indicate that
we were on scene and could have helped and were told to
leave," he said. "It crushes me if that's the case."

The CERT unit’s primary responsibility is to guard the Capitol,
though it will respond to off-ground scenes if asked. Yet the
Capitol Police source who heard the Monday request for CERT
assistance said there is some trepidation among first responders
to venture off Capitol grounds for fear of discipline.

"They were relying on our command staff to make the right
call," another Capitol Police officer said.
"Unfortunately, I don't think that happened in this case."

The US Department of Defense ordered Wednesday a security review
at its facilities worldwide, including security clearances.

"Time frame, the depth of the clearance, kind of clearance,
access, different clearances given to individuals. We're going to
look at all that," Defense Secretary Chuck Hagel said during
a news conference.

"Obviously the longer clearances go without review, there is
some jeopardy to that. There's no question about it. So we're
going to take a look at every one of those components."

Hagel has ordered the military to look at all existing security
measures, check their sufficiency, and determine what other
measures may be needed.

On Tuesday, US lawmakers demanded a review into how Alexis kept
his security clearance despite numerous violent incidents and
run-ins with the law on his record.

"I want to know who conducted his [Alexis'] background
investigation, if that investigation was done by contractors, and
if it was subject to the same systemic problems we've seen with
other background checks in the recent past," US Senator
Claire McCaskill said in a statement to Reuters.